Question

What is the relationship between one interior angle and its exterior angle?

Answer 1

Sum of an interior angle of a triangle and its exterior angle is `180^o`, so these two angles are supplementary.

Explanation:

Interior angle is formed by two sides of a polygon.
The corresponding exterior angle is formed by one of these sides and continuation of another (see the drawing.)

That makes these two angles adjacent to each other and together forming a straight angle of `180^o` degree.

Therefore, these two angles are supplementary and, added up, make `180^o`.